Certain liquids, for example liquid medication, are needed in small predetermined volumes, and therefore it is desirable to have a dispensing device for dispensing predetermined volumes of liquid. In my U.S. Pat. No. 4,892,232 I disclosed such a device.
The amount of a certain medication needed by a first patient may, however, be different from the amount of the same medication needed by a second patient. For example, the amount of Tylenol that should be administered to a child depends upon the child's weight, with a ratio of 10 milligrams per 2.2 pounds of weight (one kilogram). A forty-four pound child should therefore receive a dosage of 200 milligrams and a fifty-five pound child should receive 250 milligrams. As a child grows, the dosage of this medication that he or she should receive, therefore, grows proportionate to his or her weight change. It is desirable, therefore, to provide a dispensing valve for which the volume of liquid being dispensed can be incrementally increased or decreased across a range of volumes to accommodate such needs.
An adjustable dose dispenser having a plurality of different sized metering chambers with a stem rotatable to select the chamber to be discharged was disclosed in my U.S. Pat. No. 5,085,351. In that device, each of the metering chambers had a flexible wall. Since this device required a separate metering chamber for each volume to be dispensed, the number of selected volumes available for one valve was limited to three or four.
As an alternative to selecting one of a plurality of chambers for varying the volume of fluid to be dispensed, a device can have a single chamber where the volume dispense from the chamber is adjustable. This can be accomplished by providing a piston for dispensing the liquid from the chamber where the length of the piston stroke is adjustable, as disclosed in my previously issued U.S. Pat. No. 5,813,187. The piston operated dispensing device of U can dispense a wide range of volumes of liquid, but the device as described in my above mentioned patent has certain problems.
One problem is that the volume of liquid being dispensed is adjusted by rotating the stem of the dispensing valve and the valve stem is connected a threaded stop member which rotates with the stem to vary the stroke of the piston. To operate properly, however, the piston is sealed against the cylindrical wall of the metering chamber and the seal around the perimeter of the piston creates resistance, inhibiting the manual rotation of the dispensing stem.
Another problem relates to the structure of a piston moveable within a cylindrical wall. To prevent the twisting of the piston within the wall of the valve housing, the threads or the steps that limit the stroke of the piston should provide should provide at least two limiting stop members, with the stop members spaced evenly around the circumference of the housing. Where two such stop members are positioned in diametrically opposed positions in the housing, only 180 degrees of rotation of the stem is available to vary the length of the stroke of the piston. As a result, the variability of the adjusting quality of the valves is diminished.
Another problem is that the device includes a spring for urging the piston in a direction that maximizes the volume of the metering chamber and the other end of the spring is fitted against a surface of the housing of the valve such that on rotation of the stem either the piston rotates with respect to the spring or the spring rotates with respect to the housing. In either case the movement of the plastic of the housing or of the piston against the spring causes particles of plastic to sheer off and enter the liquid being dispensed. The continuing use of the dispenser causes those particles to obstruct the small passages that extend through the stem of the dispenser and thereby inhibit its operation.
There is therefore, a need for an improved piston operated dispensing apparatus in which the volume. of liquid being dispensed may be more easily adjusted and which will not cause small particles of plastic to be released into the liquid.